Driver arrangement for light emitting diodes

ABSTRACT

A driver arrangement for LEDs may include a PWM controller to deliver a feed voltage between an output line and a ground output line, said controller having a pin, a current regulator to regulate the feed current, a switch to connect said pin to a scaled-down version of the voltage on the first output line, and a control line to receive a signal to produce dimming, said control line coupled to said regulator and said switch to couple said regulator to a reference voltage and control switching, whereby: in one of the levels of a PWM control signal, said regulator interrupts said feed current, the voltage on said ground output line is floating, and in the other levels, said regulator is coupled to said reference voltage, said switch disconnects said pin from said voltage on the output line and the voltage on said ground output line is regulated.

FIELD OF THE INVENTION

The disclosure relates to techniques for driving Light Emitting Diodesor LEDs.

The disclosure was devised by paying specific attention to its possibleuse in driving LEDs used as lighting sources. However, reference to thispossible application is not to be construed in a limiting sense of thescope of the disclosure.

DESCRIPTION OF THE RELATED ART

Driving LEDs, especially LEDs used for lighting purposes, poses a numberof basic requirements to be met.

Compactness and high electrically efficiency of the driver may representa first requirement, in view of the expected large lots of production.High performance PWM (Pulse Width Modulation) dimming capability (e.g.LED current rise/fall times lower than, say, 5 microseconds) representsan other requirement. Very low LED current ripple (e.g. less than 1%) isanother desired feature.

High efficiency and compactness may be achieved with a single-stageswitching arrangement at the expense of high ripple current and poorfall/rise time performance. Other existing LED driver arrangements maylead to good fall/rise time performance and low ripple that exhibit poorefficiency.

In fact, Pulse Width Modulation (PWM) dimming a LED to adjust thebrightness (i.e. light intensity) thereof involves a switchingarrangement which alternatively turns the LED on and off. Adjusting theduty cycle of the on/off waveform (i.e. PWM modulating such a waveform)dictates the average power supplied to the LED and, consequently, thebrightness of the light radiation generated thereby.

Switching arrangements used to drive LEDs are inherently prone togenerating high-frequency (HF) switching ripple. A typical arrangementto avoid ripple on the current fed to a LED being dimmed is using aso-called linear current regulator, which involves deriving a feedbackcurrent signal from the feed line of the LED. This regulator has theinherent drawback of low efficiency.

Document DE-A-10 2005 028 403 discloses an arrangement which retains theadvantages of low ripple current and faster rise/fall times of linearcurrent regulators while improving the overall efficiency involvesresorting to double stage solutions involving a high-efficiencypre-regulator (DC/DC) which delivers a voltage to one or more linearcurrent regulators. A feedback signal controls the output voltage of theDC/DC stage in such a way that the voltage drop across the linearregulators is minimized. Such an arrangement involves using comparatorsto compare the voltages across the various current regulators with adefined, minimized reference value. When the voltage across a linearregulator is lower than the reference, the voltage VDD delivered by theDC/DC regulator is increased. When the output voltage is higher than thereference, the voltage VDD is decreased. This is a so called hystereticcontrol or “bang-bang” control.

OBJECT AND SUMMARY OF THE INVENTION

Despite its effectives, such a control arrangement exhibits evidentdrawbacks, especially in the case of the PWM dimmed applications. Inthat case, the current on the or each LED is chopped (on and off) by asquare wave PWM signal to change the LED brightness. During the “off”time of the PWM signal, when the voltage on the inverting inputs of thecomparators is high, a conventional hysteretic or “bang-bang” controlarrangement will tend to decrease gradually the VDD voltage. At PWM turnon, the voltage in question will generally be too low for the currentregulator to properly control the LED current.

The need is therefore felt for an improved arrangement which, inaddition to dispensing with the drawback outlined in the foregoing, mayalso meet the following requirements:

-   -   driving one or more LED modules or strings from a single voltage        source,    -   controlling LED brightness with PWM technique,    -   minimizing high frequency ripple on the LED current while        optimizing LED current rise and fall times (this is a typical        requirement for a high performance colour management system),    -   maintaining electrical efficiency as high as possible, and    -   minimizing the overall dimensions of the arrangement.

The object of the invention is thus to provide an improved arrangementwhich may properly meet the needs outlined in the foregoing.

According to the invention, that object is achieved by means of a driverarrangement having the features set forth in the claims that follow.

The claims form an integral part of the disclosure of the invention asprovided herein.

In an embodiment, a double stage (cascade) LED driver arrangement isprovided with a simple, very effective control strategy involving:

-   -   a switching stage (which by itself is very efficient and        compact), and    -   a linear stage cascaded thereto with fast rise and fall times        and a low ripple.

In an embodiment, the arrangement described herein may be controlledbased on a smart procedure in order to minimize losses without losing afast dynamic performance.

BRIEF DESCRIPTION OF THE ANNEXED DRAWING

The invention will now be described, by way of example only, withreference to the annexed FIGURES of drawing which is a block diagram ofan exemplary embodiment of the arrangement described herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description, numerous specific details are given toprovide a thorough understanding of embodiments. The embodiments can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearances of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments.

The headings provided herein are for convenience only and do notinterpret the scope or meaning of the embodiments.

The block diagram of the FIGURE of the drawing is built around threebasic components which, per se, are known in the art and are currentlyavailable at the time this application is filed.

Specifically, reference 10 denotes a DC/DC converter which is connectedbetween a constant supply voltage (for instance 24 Vcc) and the ground(0 or GND) to generate on output lines 101, 102 a feed voltage for a LEDmodule comprised of one or more LEDs (e.g. plural LEDs arranged in a LED“string”). The LED or LEDs in question are designated L as a whole.

While shown in the drawing for the sake of completeness of illustration,the LED module L does not represent per se a part of the driverarrangement which forms the subject matter of this disclosure.

More specifically, the converter 10 provides a voltage Vout on the firstoutput line 101 while the “ground” output line 102 is connected to alinear current regulator 12 which—in the exemplary embodiment shown—isbuilt around an error amplifier 120. Finally, reference 14 denotes anelectronic switch.

While a reference to these components is not to be construed in alimiting sense of the scope of the invention, exemplary componentsadapted for implementing the elements denoted 10, 12 (120) and 14 arethe components L5973AD (DC/DC regulator 10), LM2904 (error amplifier120) and MAX 4731 (switch 14).

In the drawing, the components in question are shown coupled withancillary components such as resistors, capacitors, inductors providingproper bias, filtering and so on as required for proper operation. Thoseof skill in the art will appreciate that the arrangement of ancillarycomponents as shown herein for exemplary purposes is just one of aplurality of possible arrangements. Devising alternative arrangements ofancillary components (and selecting the proper values for thesecomponents) is a design task well within the ability of the experiencedtechnician in the art and do not require a detailed description herein.

For the purposes herein, it will suffice to mention that a voltagedivider including resistors R3 and R4 is provided between the line 101(i.e. the voltage Vout) and ground (or 0 voltage). The voltage dividerin question has an intermediate point (that is the point where theresistors R3 and R4 are connected to each other) adapted to beselectively connected via the switch 14 to the feedback pin 102 of theregulator (this may be the FB pin of L5973AD), which will therefore actas a PWM controller. For the sake of clarity it will be recalled thatthe regulator 10 (e.g. L5973) provides a high frequency PWM (e.g. >50kHz) to regulate the output voltage Vout at the desired value.

This is not to be confused with the PWM dimming action proper, which iscontrolled by an external, low frequency (e.g. 100-500 Hz) PWM controlsignal derived from e.g. a manual control such as a slider actuated by auser to select a desired level of dimming of the LED module L.

The PWM control signal in question is fed over a line 16 to provide thePWM dimming action (selective on/off switching) of the LED module L.

The PWM signal provided on the line 16 controls the switch 14 and thegate of an electronic switch Q2.

The switch Q2 has the purpose of conveying towards the non-inverting (+)input of the error amplifier 120, alternatively, a non-zero nominalcurrent reference value (e.g. 700 mA) or a zero value (0 mA) as afunction of the value of the PWM control signal applied on the line 16.

To that effect, another voltage divider comprised of two resistors R10and R11 is arranged between a reference voltage Vref and ground (0voltage) with an intermediate point (i.e. the point where the resistorsR10 and R11 are connected to each other) connected to the non-invertinginput of the error amplifier 120.

In an embodiment, the control switch Q2 is a mosfet whose drain-sourceline is connected across the resistor R10. The switch Q2 thus causes areference voltage to be generated at the non-inverting input of theerror amplifier 120, which reference voltage is a function of anexternal reference voltage Vref and the values of R10 and R11.

The switch Q2 thus controls via the error amplifier 120 the gate ofanother electronic switch Q1 (again a mosfet in an embodiment) which isarranged between the line 102 (i.e. the voltage V) and a shunt currentmeasurement resistor Rm.

Reference 18 denotes the feedback line of the linear current regulator12 which senses via the sensing resistor Rm voltage representative ofthe current flowing through the switch Q1 (which corresponds to thecurrent flowing through the LEDs) and feeds it back to the invertinginput of the error amplifier 120.

Essentially, the role of the error amplifier 120 is to maintain the LEDcurrent regulated at the value specified on the non-inverting input byacting on the power mosfet Q1 according to a typical feedback scheme. Ifthe current measured (i.e. sensed) is lower than the desired value forthe current, the error amplifier 120 increases the opening of thechannel of the power mosfet thus increasing the current through theLEDs. Conversely, if the current sensed is higher than the desired valuefor the current, the error amplifier 120 decreases the opening of thechannel of the power mosfet thus reducing the current through the LEDs

Operation of the arrangement shown in the drawing is as follows.

When the PWM signal on the line 16 is high, the voltage at thenon-inverting input of the error amplifier 120 is equal to zero and themosfet comprising the switch Q1 is open circuit, so that the current onthe LED module L is equally zero (“off” portion of the PWM dimmingaction of the LED module) and the current regulator is practicallyde-activated. The voltage Vr on the “ground” line 102 is floating. Thelogical switch 14 is closed and connects the feedback pin 104 of the PWMcontroller 10 to a voltage as Vpart present ad the intermediate pointbetween resistors R3 and R4 in the voltage divider formed thereby. Thevoltage Vpart is a scaled-down version of the output voltage Vout on theline 101.

In this state, the voltage Vout is regulated at a defined value that isset to be high enough, i.e. higher than a threshold selected to ensurethat the linear current regulator 12 will work properly at thesubsequent PWM turn on.

When the PWM on the line 16 is low, the current regulator 12 isactivated (“on” portion of the PWM dimming action of the LED module).The current regulator controls the LED current at the desired value Irefas set by reference voltage Vref and the values of the resistors R10,R11. At the same time, the switch 14 will be open and the feedback pin104 of the PWM controller 10 will be disconnected from the voltagedivider R3, R4 (and thus rendered independent of Vout).

In this state, the voltage Vr on the “ground” line 102 will be activelyand dynamically regulated at a level (Vrl) which is selected to be thelowest possible value that allows at the same time to minimize the powerlosses in the linear regulator 12 while still ensuring LED currentregulation.

It will be appreciated that the relationship between the “high” and“low” states of the PWM signal on the line 16 and the “off” and “on”portions of the PWM dimming action of the LED module may be the oppositeto the one referred to in the foregoing, due e.g. to the presence of alogical inverter associated with the line 16. For that reason, one andthe other of the on/off levels of the PWM control signal are recited inthe claims that follow.

Additionally, it will be appreciated that the designation of the outputline 102 of the controller 10 as a “ground” line is related to thenotional role of that line with respect to the first output line 101(which is brought to a voltage Vout). As described, the voltage Vr onthe line 102 is not fixed to ground (i.e. to zero), but is eitherfloating or regulated to a minimum value depending on the operatingconditions of the arrangement.

The arrangement described herein may be based on a more reliable, lessnoisy and fast dynamic error amplifier (as possibly integrated in apre-regulator such as 5973AD) in the place of hysteretic control withexternal comparators.

Without prejudice to the underlying principle of the invention, thedetails and embodiments may vary, even significantly, with respect towhat has been described and shown by way of example only, withoutdeparting from the scope of the invention as defined by the annexedclaims.

1. A driver arrangement for light emitting diodes, the driverarrangement comprising: a high frequency PWM controller to deliver alight emitting diode feed voltage between a first output line and aground output line, said controller having a PWM controller feedbackpin, a linear current regulator coupled to said ground output line ofsaid controller to regulate the light emitting diode feed current, saidcurrent regulator selectively couplable to a reference voltage, aconnection switch selectively actuatable to connect said feedback pin ofsaid controller to a scaled-down version of the voltage on the firstoutput line of said controller, and a PWM control line to receive anon/off PWM control signal to selectively produce controlled lightemitting diode dimming, said PWM control line coupled to said currentregulator and said connection switch to selectively couple said currentregulator to said reference voltage and control switching of saidconnection switch, whereby: in one of the on/off levels of said PWMcontrol signal, said current regulator is configured to interrupt saidlight emitting diode feed current, the voltage on said ground outputline of said controller is floating, and said connection switch connectssaid PWM controller feedback pin to said scaled-down version of thevoltage on said first output line of said controller, and in the otherof the on/off levels of said PWM control signal, said current regulatoris coupled to said reference voltage and controls said light emittingdiode feed current, said connection switch disconnects said feedback pinof said controller from said scaled-down version of the voltage on thefirst output line of said controller and the voltage on said groundoutput line of said controller is regulated.
 2. The arrangement of claim1, further comprising: a control switch connected to said PWM controlline to cause said current regulator to be coupled to either of anon-zero reference voltage and a zero reference value.
 3. Thearrangement of claim 1, wherein said controller is configured toregulate the voltage of said first output line at a level higher thanthe threshold or the sum of the thresholds of the light emitting diodeor light emitting diodes driven by the arrangement while said PWMcontroller feedback pin is connected by said connection switch to saidscaled-down version of the voltage on said first output line of saidcontroller, thus allowing said current regulator to operate at asubsequent PWM turn on.
 4. The arrangement of claim 1, wherein saidcontroller is configured to regulate the voltage on said ground outputline at a given value while said current regulator is activated.
 5. Thearrangement of claim 4, wherein said given value is a minimum value tominimize power losses in said current regulator while permitting lightemitting diode current to be regulated by said current regulator whenactivated.